NO EFFECT OF AEROBIC FITNESS ON EXERCISE-INDUCED DIAPHRAGM FATIGUE IN FEMALES.

We tested the hypothesis that the incidence and magnitude of diaphragm fatigue following high-intensity exercise would be lower in females with a high aerobic capacity (Hi-Fit) compared to healthy females with an average aerobic fitness (Avg-Fit). Participants were assigned to groups based on their peak O2 uptake (V̇O2peak) obtained during cycle exercise: Hi-Fit n=9, V̇O2peak > 56.1±3.4 mL·kg-1·min-1 vs. Avg-Fit n=9, V̇O2peak < 35.7±4.9 mL·kg-1·min-1. On a second day, diaphragm fatigue was assessed before and after constant load exercise test to exhaustion. Magnetic stimulation of the phrenic nerve roots was used to non-volitionally assess diaphragm fatigue by measurement of transdiaphragmatic twitch pressure (Pdi,tw). Both groups exercised at > 90% of V̇O2peak for a similar duration (Hi-Fit: 546.1 ± 177.8 vs Avg-Fit: 559.3 ± 175.0 seconds, p = 0.9). Diaphragm fatigue was defined as a > 15% reduction in Pdi,tw, approximately 2 times greater than the coefficient of variation. The mean group average reduction in Pdi,tw following exercise in the Hi-Fit (17.5%) and Avg-Fit groups (12.2%) was not different between groups (p = 0.2). The Hi-Fit group performed exercise at a higher absolute work rate that elicited significantly greater ventilatory work and inspiratory muscle force output. The Hi-Fit group did not experience greater fatigue compared to the Avg-Fit group, which we attribute to a greater reliance on accessory respiratory muscle recruitment, to training-induced increases in the aerobic capacity of the diaphragm, or a combination of the two. In summary, aerobic fitness is not predictive of exercise-induced diaphragm fatigue in healthy females.

The human skeletal muscle metaboreflex contribution to cardiorespiratory control in males and females in dynamic exercise.

There is a significant effect of sex and muscle mass on the cardiorespiratory response to the skeletal muscle metaboreflex during isometric exercise. We therefore tested the hypothesis that sex differences would be present when isolated following dynamic exercise. We also tested the hypothesis that single and double leg post-exercise circulatory occlusion (PECO) following heavy exercise would elicit a cardiorespiratory response proportional to the absolute muscle mass. Healthy (24 ± 4 years) males (n = 10) and females (n = 10) completed pulmonary function and an incremental cycle test to exhaustion. Participants completed two randomized, 6 min bouts of intense cycle exercise (84 ± 7% V̇O2peak). One exercise bout was immediately followed by 3 min PECO (220 mmHg) of the legs while the other exercise bout was followed by passive recovery. Males completed an additional session of testing with single leg PECO. The mean arterial pressure during PECO and control was greater in males compared to females (p = 0.004). The was a significant time by condition by sex interaction in the heart rate response to PECO (p = 0.027). There was also a significant condition by sex interaction in the ventilatory response to PECO (p = 0.026). In males, we observed a dose-dependent cardiovascular, but not ventilatory, response to muscle mass occluded (all p < 0.05). Our findings suggest the metaboreflex contribution to cardiorespiratory control during dynamic exercise is greater in males compared to females. The ventilatory response induced by double-leg occlusion but not single-leg occlusion, suggests that the ventilatory influence of the metaboreflex is less sensitive than the cardiovascular response and may be linked to the greater afferent activation induced by double-leg occlusion.